You only need to change UCFG1 if the default settings are not what you want. If they are ok, you can ignore UCFG1.

To change UCFG1 you have two choices:

1. Choose Device Configuration... from the ISP menu in Flash Magic

2. Add a hex record using locations FFF0H - FFF8H to your hex file. See Keil's start900.a51 for details on which byte is used for UCFG1.

Note that changes to UCFG1 only take effect after a reset.

Note that if you change the clock source to an external crystal or clock, make sure you have one connected, otherwise your device won't start up again.

Andy